1. Field of the Invention
The present invention relates to an ink jet print head having a plurality of ejection openings to eject yellow, magenta and cyan inks, an ink jet printing apparatus using the print head, and an ink jet printing method.
2. Description of the Related Art
As personal computers, word processors and facsimile machines have come into widespread use in offices and homes in recent years, a variety of types of printing apparatus have been commercialized as information output devices for these apparatus. Of these output devices an ink jet printing apparatus has many advantages, such as the capability to perform color printing relatively easily using a plurality of color inks, low printing noise, the capability to print on a variety of print media with high print quality and small printer size. Because of these advantages, an ink jet printing apparatus is suited for office use and for personal use at homes. Of the ink jet printing apparatus, a serial type that reciprocally moves the print head over a print medium for printing is very popular because of its ability to perform high quality, low cost printing.
In a serial type printing apparatus, bidirectional printing may be performed in which a print head mounted on a carriage (and capable of ejecting a plurality of inks) prints an image as the carriage moves forward and backward. In such a printer the order in which colored inks are ejected (the ink ejection order) is reversed for forward and backward movement of the print head. The reversal of the ink ejection order between the forward and backward movements of the carriage may cause bandlike grayscale level variations on a printed image (hereinafter referred to also as a “bidirectional color difference”).
A print head described in Japanese Patent Laid-Open No. 2004-001491, for example, has five ink ejection opening arrays, of which one is for yellow ink, two for cyan ink and the remaining two for magenta ink. The yellow ink ejection opening array is situated at the center of the five ejection opening arrays, with one cyan ink ejection opening array and one magenta ink ejection opening array on each side thereof. Two sets of the cyan ink ejection opening array and the magenta ink ejection opening array are arranged symmetrical with respect to the center yellow ink ejection opening array so that the ink ejection orders during the forward and backward movements are the same.
In such a print head, the yellow ink ejection opening array has only those ejection openings that eject large ink droplets while the cyan and magenta ink ejection opening arrays each have ejection openings that eject large and small ink droplets. This arrangement is made because the bright yellow ink hardly shows graininess of an image even if printed with only large ink droplets, whereas the cyan and magenta inks with low brightness levels tend to show graininess in printed areas with low grayscale levels unless the image is printed with small ink droplets. Therefore, small yellow ink droplets are not necessary to reduce graininess. So, only large ink droplet ejection openings are provided for yellow ink.
There has been a growing call for faster printing speeds in recent years and, among methods known to meet that demand, there is one method that reduces the number of scans (scan number). In printing images with the same grayscale levels, a reduction in the number of scans can increase ink ejection density in one scan.
In Japanese Patent Laid-Open No. 2004-001491, a yellow ink ejection opening array has large ink droplet ejection openings arrayed on each side of a corresponding ink supply port. The magenta and cyan ink ejection opening arrays each have large ink droplet ejection openings arrayed on one side of a corresponding ink supply port and small ink droplet ejection openings arrayed on the other side thereof. So, magenta and cyan large ink droplets are ejected from the openings situated on one side of the corresponding ink supply ports while yellow large ink droplets are ejected from openings situated on both sides of the corresponding ink supply port. Therefore, the print head has a high ejection density in a unit area for the large yellow ink droplets and, when an image of high grayscale level is printed, a strong air flow is produced near the yellow ink ejection opening array by the ejection of large yellow ink droplets. This raises a possibility of bandlike color variations being produced when an image of secondary color is printed by ink droplets ejected from the yellow ink ejection opening array and the adjoining magenta or cyan ink ejection opening array. That is, the air flow produced near the former ejection opening array may disturb landing positions of ink droplets ejected from the latter ejection opening array, causing bandlike color variations. Such bandlike color variations are called “air flow-based color variations”. Small ink droplets are affected particularly strongly by the air flows.